1. Field of the Invention
The present invention relates to a semiconductor device formed by a thin film transistor (hereafter referred to as TFT) using a semiconductor film as an active layer, and to a manufacturing method thereof. Specifically, the present invention relates to laser crystallization of a semiconductor film by laser annealing.
2. Description of the Related Art
An advance has been made in recent years in development of thin film transistors (hereinafter referred to as TFTs) using semiconductor films as active layers, and TFTs using polycrystalline silicon films (polysilicon films) as crystalline semiconductor films are receiving the attention. In liquid crystal display devices (liquid crystal displays) and EL (electroluminescence) display devices (EL displays), in particular, such TFTs are used as elements for switching pixels and elements for forming driver circuits to control the pixels.
General means for obtaining a polysilicon film is a technique in which an amorphous silicon film is crystallized into a polysilicon film. A method in which an amorphous silicon film is crystallized with the use of laser light has lately become the one that is especially notable. In this specification, to crystallize an amorphous semiconductor film with laser light to obtain a crystalline semiconductor film is called laser crystallization.
The laser crystallization is capable of instantaneous heating of semiconductor film, and hence is an effective technique as measures for annealing a semiconductor film formed on a low heat resistant substrate such as a glass substrate or a plastic substrate. In addition, the laser annealing makes the throughput definitely higher as compared with conventional heating measures using an electric furnace (hereinafter referred to as furnace annealing).
There are various kinds of laser light, of which the general one to be used in laser crystallization is laser light generated and emitted from a pulse oscillation type excimer laser as a source (hereinafter referred to as excimer laser light). The excimer laser has advantages in that it is large in output and that it is capable of repetitive irradiation at a high frequency and, moreover, excimer laser light is advantageous in terms of its high absorption coefficient with respect to silicon films.
The problem drawing the most attention at present is whether or not the grain size of a crystalline semiconductor film crystallized by laser light can be made larger. Naturally, if one grain becomes larger, in particular the number of grain boundaries crossing a channel forming region of a TFT is reduced. It is therefore possible to improve the dispersion in the electric field effect mobility and the threshold voltage, typical electrical characteristics of the TFT.
Further, relatively clean crystallinity within each grain is maintained, and in order to raise the above stated TFT characteristics, it is preferable to form the TFTs so as to have the channel forming region completely contained with one grain.
However, it is difficult to obtain a crystalline semiconductor film having a sufficiently large grain size with current techniques, and although there are reports of such films being obtained experimentally, at present this has not reached a level of practical use.